The optical beam-switching chopper of the invention is useful in many different optical applications, but was developed particularly for use in a double-beam atomic absorption spectrophotometer which requires the switching of two separate light beams into two separate paths in a particular cyclical sequence so that the detector electronic circuitry can accurately determine an absorption value independently of variations in light beam intensity or detector sensitivity or background absorption that may be caused by molecular absorption or scattering.
In atomic absorption spectrometry, a sample substance is brought into a substantially atomic state, for example, by spraying a solvent containing the element into a flame in the spectrophotometer atomizing furnace. A sample light beam, which originates from a line-emitting light source, such as a hollow cathode tube, and which includes a resonance line of the element to be measured, is directed through the furnace. The desired element in the sample absorbs the resonance lines characteristic of the element and the emerging light beam is directed through a monochromator and thence to a detector which measures the degree to which the desired element absorbs the resonance lines of the sample beam. This absorption degree represents the amount of desired element in the sample substance.
To correct for lamp and detector variations that would obviously cause erroneous measurements, a double beam form the line-emitting source is generally used. One beam passes through the atomic furnace where the element to be measured absorbs its characteristic resonance lines; the other beam, from the same source, bypasses the furnace and is applied directly to the common detector. The ratio of the two signals detected thereby give an indication of the absorption by the element independently of variations in light source or detector sensitivity.
Unfortunately, other error-generating factors exist that cannot be overcome by the above described double-beam system. In practice, other elements in the sample may partially absorb a resonant line and also various components in the burning sample may cause a light scattering. Such scattering and losses due to molecular absorption are designated as "background absorption", and may be measured with a second double beam that originates from a light source having a continuous spectrum, such as a deuterium lamp. This continuous spectrum source follows the same double path and, by comparison of the double path line-emitting beam with the double path continuous spectrum beam, the electronics associated with the detector can readily determine and correct for the background absorption.
This invention is for an efficient optical beam-switching chopper which receives the light beams originating from the continuous spectrum source and from the line-emitting source and, in a cyclical sequence, directs first the resonance line-emitting beam into the atomized sample path, then the continuous source beam into the sample path, followed by the line-emitting beam into the reference path, and then the continuous spectrum beam into the reference path. Alternatively, the chopper is arranged for first directing the resonance line-emitting beam into the atomized sample path and then into the reference path, followed by directing the continuous source beam into the atomized sample path and then into the reference path.
Briefly described, the optical chopper of the invention includes the first rotating chopper disc having two reflecting sectors, each approximately 90.degree. and spaced between open or transparent sectors; and a second rotating disc having four reflecting sectors, each of approximately 45.degree. and spaced between similar open sectors. It is noted that the first rotating chopper disc could have more or less than two reflecting sectors, as long as the second rotating chopper disc has twice as many reflecting sectors as the first rotating chopper disc. The two chopper discs are mounted on a common rotatable shaft and are spaced apart with their reflecting surfaces facing each other. The light beam from a first light source enters the chopper assembly and is folded by a plane mirror between the segmented discs to the reflecting plane of the second or 4-sector disc, while the beam from the second light source is admitted to the chopper at the rear side of the second disc and at a point corresponding to the precise location that the first light source beam strikes the opposite surface. Thus, if there is a reflecting surface at that point, the first light beam will be reflected back toward the first 2-sector disc while the second beam will be absorbed; if there is an open segment, the second light beam will continue toward the 2-sector disc and the first beam will be lost.
The particular beam selected by the position of the 4-sector disc is directed to the reflecting plane of the rotating 2-sector disc. If the beam sees an open section, it passes from the chopper along a first path. If the beam sees an open section, it passes from the chopper along a first path. If the beam strikes a reflecting surface on the 2-sector disc, it is reflected back to a plane mirror which reflects the beam into a second path. Both plane mirrors between the sector discs are positioned with respect to the disc surfaces so as to direct the beams at a favorable angle that optimizes the chopper efficiency.
Alternatively, to change the sequence of operation, the positions of the first and second rotatable chopper discs could be interchanged. Also, in one form of the invention, during one segment of the chopper cycle both beams are blocked from both output paths.